1. Field of the Invention
The present invention relates to a light source for a microwave-powered lamp.
2. Description of the Related Art
It is known to excite a discharge in a capsule with a view to producing light. Typical examples are sodium discharge lamps and fluorescent tube lamps. The latter use mercury vapour, which produces ultraviolet radiation. In turn, this excites fluorescent powder to produce light. Such lamps are more efficient in terms of lumens of light emitted per watt of electricity consumed than tungsten filament lamps. However, they still suffer the disadvantage of requiring electrodes within the capsule. Since these carry the current required for the discharge, they degrade and ultimately fail.
We have developed electrodeless bulb lamps, as shown in our patent application Nos. PCT/GB2006/002018 for a lamp (our “'2018 lamp”), PCT/GB2005/005080 for a bulb for the lamp and PCT/GB2007/001935 for a matching circuit for a microwave-powered lamp. These all relate to lamps operating electrodelessly by use of microwave energy to stimulate light emitting plasma in the bulbs. Earlier proposals involving use of an airwave for coupling the microwave energy into a bulb have been made for instance by Fusion Lighting Corporation as in their U.S. Pat. No. 5,334,913. If an air wave guide is used, the lamp is bulky, because the physical size of the wave guide is a fraction of the wave length of the microwaves in air. This is not a problem for street lighting for instance but renders this type of light unsuitable for many applications. For this reason, our '2018 lamp uses a dielectric wave-guide, which substantially reduces the wave length at the operating frequency of 2.4 Ghz. This lamp is suitable for use in domestic appliances such as rear projection television.
In our International Application No. PCT/GB2008/003829, now published under No. WO 2009/063205, we provide a light source to be powered by microwave energy, the source having:                a solid plasma crucible of material which is transparent or translucent for exit therefrom, the plasma crucible having a sealed void in the plasma crucible,        a Faraday cage surrounding the plasma crucible, the cage being at least partially light transmitting for light exit from the plasma crucible, whilst being microwave enclosing,        a fill in the void of material excitable by microwave energy to form a light emitting plasma therein, and        an antenna arranged within the plasma crucible for transmitting plasma-inducing microwave energy to the fill, the antenna having:        a connection extending outside the plasma crucible for coupling to a source of microwave energy;the arrangement being such that light from a plasma in the void can pass through the plasma crucible and radiate from it via the cage.        
As used in that application and this specification:
“lucent” means that the material, of which the item described as lucent, is transparent or translucent;
“plasma crucible” means a closed body enclosing a plasma, the latter being in the void when the latter's fill is excited by microwave energy from the antenna.